Tired of manually setup test data of Kotlin data classes or POJOs? Instantiator creates Instances of any class for you so that you can focus on writing tests instead of spending time and effort to set up test data.

This is not a mocking library. When talking about Instantiator can create an instance of any class for you, I'm referring on data class or POJOs, not mock functionality.

It doesn't use any black magic. It uses reflection and invokes the public primary constructors. If there is no public primary constructor available, then Instantiator cannot instantiate it.

testImplementation 'com.hannesdorfmann.instantiator:instantiator:0.3.0'

or SNAPSHOT (directly built from main branch):

testImplementation 'com.hannesdorfmann.instantiator:instantiator:0.3.1-SNAPSHOT'

Assuming you have some data structures like this

data class Person(
    val id: String,
    val firstname: String,
    val lastname: String,
    val age: Int,
    val gender: Gender
)

enum class Gender {
    MALE,
    FEMALE,
    UNDEFINED
}

fun computeFullname(p: Person): String {
    return "${p.firstname} ${p.lastname}"
}

And you want to write some (unit) tests to test some functionality you often have to manually set up some object instances just for testing. This is where Instantiator comes in: it creates such instances for you.

@Test
fun someTest() {
    val person: Person =
        instance() // instance() is Instantiator's API. It creates a new instance of Person with random values. 

    // Now that you have some random Person object 
    // you can do with it whatever you want
    val expected = "${person.firstname} ${person.lastname}"
    val fullname = computeFullname(person)
    assertEquals(expected, fullname)
}

Bonus: if you want to get an instance for each subclass of a sealed class hierarchy use instantiateSealedSubclasses() like this:

sealed class Root
data class A(i: Int) : Root()
data class B(i: Int) : Root()


sealed class NestedRoot : Root()
data class N1(i: Int) : NestedRoot()


val subclassesInstances: List<Root> = instantiateSealedSubclasses()
println(subclasses) // contains 3 instances and prints  "A, B, N1"

You can configure Instantiator by passing a InstantiatorConfig instance as parameter to instance(config : InstantiatorConfig) or instantiateSealedSubclasses(config : InstantiatorConfig).

Some settings that you can set:

• InstantiatorConfig.useDefaultArguments: Set it to true if you want that the default arguments of constructor parameters are used if provided. For example, given data class MyClass(val id : Int, val name : String = "Barbra"), if config.useDefaultArguments = true then the parametername of any instance will be name="Barbra" and only id which has no default argument set will be filled with random value. Default value of the default config is that InstantiatorConfig.useDefaultArguments = true.
• InstantiatorConfig.useNull: Set it to true if for constructor parameters that can be null, null is actually the value. for example, given data class MyClass(val id : Int?), if config.useNull = true then instance will look like MyClass( id = null). If config.useNull = false then nullable parameters will have non null values i.e. MyClass ( id = 123). Default value of default config is InstantiatorConfig.useNull = true

In case a constructor parameter is both, nullable and has a default value, then the default config uses the default value for the parameter. Example:

class Foo(val i: Int? = 42)

val foo = instance<Foo>()

println(foo.i) // prints 42

InstantiatorConfig takes as a constructor parameter vararg factories: InstanceFactory. An InstanceFactory is used to create an instance in case a unsupported type needs to be instantiated (see Supported use cases table above) or if you want to override how primitive types are instantiated.

class MyIntInstanceFactory : InstantiatorConfig.InstanceFactory<Int> {
    override val type: KType = Int::class.createType()
    override fun createInstance(): Int = 42
}

val config = InstantiatorConfig(MyIntInstanceFactory())

val i = instance<Int>(config) // i == 42 and all other ints will be 42 when using this config

InstantiatorConfig is immutable. You can add an InstanceFactory with the val newConfig : InstantiatorConfig = instantiatorConfig.add(myCustomFactoy)